The present invention relates generally to dielectric filters, and, more particularly, to a dielectric filter construction which forms a one half-wave wavelength resonator.
The design and use of filter circuitry for filtering a signal of undesired frequency components is well known. For example, filter circuitry for performing bandpass, band reject, low pass, and high pass functions are all well-known, and are utilized to form portions of electrical circuits. Combinations of such filter circuits are additionally well-known and are utilized to form portions of electrical circuits. Such filter circuitry permits passage of, or rejection of, certain frequency component portions of a signal applied to the filter circuitry. The component portions of the signal applied to the filter which are passed, or rejected, by the filter is, of course, a function of the characteristics of the filter.
Filter circuitry may be formed of either active or passive filter components. Active filter components are advantageously utilized to embody the filter circuitry within an integrated circuit. However, filter circuitry comprised of active filter components is generally linear over only a limited dynamic range. Additionally, filter circuitry comprised of active filter components exhibit desired filter characteristics over only the limited dynamic range.
Filters comprised of passive filter components are therefore commonly utilized to embody the filter circuitry. Passive filter components of which the filter circuitry may be comprised include for example, combinations of resistors, capacitors, and inductors. The resistive, capacitive, and inductive component values of such passive filter components, and their respective electrical connections therebetween, define a resonant frequency. The passive filter components may be connected in manners, and may be of resistive, capacitive, and inductive values, to form any of the above-listed types of filter circuitry.
Filter circuitry forming a portion of an electrical circuit may, for example, be positioned in a series connection with the electrical circuit. When signals generated by, or applied to, the electrical circuit are supplied to series-connected filter circuitry, signal portions (i.e., frequency component portions) of the signal applied to the filter circuitry within the resonant frequency defined by the component values of component portions of the filter circuitry are passed therethrough. Appropriate selection of the component values of passive filter components, as well as their electrical connection therebetween, causes the filter circuitry to pass, or to reject, signal portions of any selected range of frequencies.
Filter circuitry forming a portion of an electrical circuit may, conversely, be positioned in a shunt connection with other portions of the electrical circuit (i.e., the filter circuitry may be positioned to extend between the electrical circuit and a ground plane). Similar to the series-connected filter circuitry, the values of the passive filter components, and their respective electrical connections therebetween, define a resonant frequency. When the filter circuitry is connected to the electrical circuit in such a shunt connection, signal portions (i.e., frequency component portions), of a signal applied to the filter circuitry within the resonant frequency of the filter circuitry are shunted to ground by filter circuitry. By appropriate selection of the component values of the components of the filter circuitry, as well as their respective electrical connection therebetween, any of the above-listed circuitry may be formed.
Combinations of both filter circuitry connected in the series-connection and the shunt-connection may, of course, be formed, to perform circuit functions as desired.
A radio frequency receiver circuit comprises one type of electrical circuit which utilizes filter circuitry to form a portion thereof. Such filter circuitry is utilized, for example, to tune the receiver, and to filter intermodulation spurs generated during down conversion and demodulation of a signal received by the receiver circuit. Actual, non-ideal receiver circuits generate intermodulation distortion during down conversion of the received signal. Additionally, spurious signals are generated during down conversion of a signal received by such a non-ideal receiver circuit. Filter circuitry is utilized to reject such intermodulation distortion generated during the down-conversion and/or demodulation process. Filter circuitry is, of course, utilized in receiver circuits to perform other filter functions.
Passive filter circuits are oftentimes comprised of ceramic and other dielectric materials. Such filter circuitry is commonly referred to as a "ceramic block filter" because of the geometric configuration of most of such filters. Conventionally, the ceramic block filter is formed in the shape of a block, and one or more holes are drilled or otherwise formed to extend into the block. Such holes (i.e., cavities) form resonating cavities which resonate at frequencies determined by the length of the cavity. Portions of the sidewalls defining the cavity are coated with an electrically-conductive material, such as a silver-containing compound. Portions of surfaces, or entire surfaces, of the ceramic block are also typically covered with the electrically-conductive material.
The surface area of the sidewalls which define the cavities additionally determine the resonating frequency of the resonator formed therefrom. Holes may be drilled (i.e., the cavities may be formed) to extend in any direction. Typically, however, the holes are formed to extend between opposing surfaces of the ceramic block, such as, for example, between top and bottom surfaces, or between front and rear surfaces of the ceramic block. The ceramic block filter may be connected in series, or in shunt, to perform filter functions as desired. Ceramic block filters and/or apparatus for connecting such filters to an electrical circuit are disclosed in U.S. Pat. Nos. 4,431,977; 4,673,902; 4,703,921; 4,716,391; and 4,742,562.
Because many electrical devices are packaged in ever-smaller housings, the electrical circuit comprising portions of the electrical devices must be miniaturized to permit positioning of the electrical circuits within the ever-smaller housings.
For example, portable transceivers, such as portable, cellular phones, are increasingly miniaturized to permit the transceiver to be of ever smaller dimensions. Electrical circuits of such portable transceivers include both receiver circuitry and transmitter circuitry each of which may utilize one or more ceramic block filters for filtering signal portions of signals received by the receiver circuitry, and for filtering signal portions of the signals generated by the transmitter circuitry. The ceramic block filters may, for instance, form interstage filters positioned between stages of the transmitter and/or receiver circuitry, or form a duplexer filter positioned between the receiver circuitry and an antenna and between the antenna and the transmitter circuitry.
Typically, the ceramic block filter is mounted upon a circuit board, such as a printed circuit board, and is suitably connected to an electrical circuit disposed, or mounted, thereupon. Because of the geometric configuration of the ceramic block filter, a minimum heightwise spacing is required above the circuit board to permit mounting of the ceramic block filter thereupon. More particularly, when the circuit board upon which the filter is mounted to be positioned with a transceiver housing, the circuit board must be positioned a distance at least as great as the distance of such minimum heightwise spacing beneath the inner surface of the housing of the transceiver. Similarly, when two or more circuit boards are to be stacked upon one another, the distance between the circuit boards must similarly be at least as great as such minimum heightwise spacing. This heightwise spacing necessitated by the geometric configuration of the ceramic block filter may limit the miniaturization permitted of an electrical device, such as the portable transceiver as above-mentioned.
Various means have been suggested for reducing the minimum heightwise distance required for mounting a ceramic block filter upon a circuit board.
Most simply, the dielectric block filter may be positioned upon the circuit board such that the axially extending resonators formed to extend through at least portions of the dielectric block filter, extend in directions parallel to the planar direction of the circuit board. However, such positioning of the dielectric block filter requires significant amounts of surface area of the circuit board to be positioned in such a manner. When the resonators formed to extend through the dielectric block are of lengths corresponding to a one half-wavelength--i.e., one half of the wavelength of the resonating frequency of the resonator, the surface area required for such positioning of the dielectric block filter is particularly significant. For instance, when the resonating frequencies of the resonators are to be approximately 900 MHz, the length of the resonating cavities are approximately sixteen and one half centimeters in length.
Additionally, U.S. patent application Ser. No. 455,062, filed on Dec. 22, 1989, discloses a dielectric block filter which is of dimensions permitting the positioning thereof through a opening formed to extend through a circuit board. A bracket is positioned about the ceramic block filter to affix the filter to the circuit board. Also, U.S. patent application Ser. No. 07/577,172 filed Sept. 4, 1990 to Michael T. Metroka discloses a dielectric block filter which may be similarly positioned to extend into an opening formed through the circuit board, but which obviates the need of a bracket to affix the filter to the circuit board.
However, such dielectric block filter constructions typically require a shielding bracket to be positioned at an end surface of the dielectric block to prevent radiation emitted through an end portion of the dielectric block from interfacing with operation of other portions of the electrical circuit, or other electrical circuits. The shielding bracket, comprised of a metallic material, is required to cover the end portion of the dielectric block to prevent transmission of electromagnetic waves from an exposed surface of the dielectric block. Such transmission of electromagnetic waves would otherwise interfere with circuit operation of electrical circuits positioned proximate to the dielectric block filter. Such shielding brackets, however, necessitate additional surface area of the circuit board, and, additionally, require an extra production step to position the bracket about the end surface of the dielectric block filter during mounting thereof upon the circuit board.
What is needed, therefore, is a dielectric filter construction which forms a one half-wave wavelength resonator and which obviates the need of a shielding bracket formed about an end surface thereof.